Ground circuit supervisory system



April 1963 M. J. KRAMER 3,086,999

GROUND emcuzw SUPERVISORY SYSTEM Filed April 26, 1960 2 Sheets-Sheet 1 j INVENTOR. flax J Kramer Mmni 6%.4,

ATTORNEY April 3, 1963 M. J. KRAMER GROUND CIRCUIT SUPERVISORY SYSTEM 2 Sheets-Sheet 2 Filed April 26, 1960 ATTbRn/EY 3,ti86,99 GRGUND CERCUET SUPERVESQRY EYS EEM Max J. Kramer, Parma, (this, assignor to Aiuminurn Company of America, Pittsburgh, Pa, a corporaticn of Pennsylvania Filed Apr. 26, 19nd, Ser. No. 24,812 8 Ciaims. (Cl. 1 3 124) This invention relates to electrical systems, and is applicable generally to the supervision of the condition of earth ground circuits, especially of safety ground circuits provided for electrical equipment for affording protection to personnel against electrical hazards.

In the installation and operation of industrial electrical apparatus and devices subject to commercial power potentials, safety grounding through a low resistance ground wire or cable is employed to prevent exposed metallic parts of the apparatus from becoming electrically charged substantially above ground potential. The purpose primarily is to promote personnel safety, but it can unknowingly be defeated by accidental breakage of the ground circuit, or through resistance build-up due to deteriorated cable and poor terminal connections. In various instances, detrimental operating and corrosive ambient conditions are present to intensify the problem of having an effective safety ground. A particular case in point relates to molten metal treating and transfer ladles having removable covers provided with electrical heating elements and requiring use of detachable or plug type connectors for the electrical circuits to permit disconnection thereof when a cover is to be moved between open and closed relationship with respect to its ladle. The heating element is usually suspended from the underside of the cover and energized from a 220 or 440 volt power circuit, and under prevailing operating conditions, it is extremely difficult to maintain adequate voltage insulation between the heating element and the metal shell of the cover. There is thus present in the use of such equipment the augmented liability of a ground circuit becoming defective and failing to provide an effective safety ground. In some cases a ground cable separate from the plug connector is provided, so there eX- ists the possibility of an operator failing to reconnect the ground circuit with the cover, due to the proneness of the human factor or err. The importance of a safety ground supervisory system, or What might be termed a safety ground tell-tale or relay, is readily apparent.

One object of my invention, therefore, is to provide an improved and effective system or apparatus for testing and monitoring a ground loop circuit for determining its resistance and continuity.

Another object is to provide a ground circuit supervisory system which is inductively operatively connected to a ground loop circuit and which will respond to detrimental change in the condition of such circuit and give an alarm and/or disconnect the ground circuit-protected electrical device or apparatus from its power supply source.

Another object is to provide a ground circuit-condition detecting system which is self-testing and operable in response to any detrimental change in the condition of the ground circuit or to the component parts of the system, so that upon occurrence thereof the fact will be made known by signal or by power disconnection.

Still another object is to provide a safety ground relay which includes a ground loop circuit serving an electrical power consuming device and which relay is characterized by its prevention of power flow to such device until said ground circuit is completed and is in effective operating condition for its intended purpose.

These and further objects and advantages of the invention will become apparent from the following detailed description of a particular embodiment thereof, taken in conjunction with the accompanying drawings.

in carrying out my invention in a preferred form, 'I provide two saturable reactors or single core magnetic amplifiers and provide them with bias windings and alternately pulsed gate or load windings, and dispose them in inductive relation with a low resistance ground loop circuit so that their cores will be driven to cut-off by the bias windings and prevent firing for minimum load current flow through the gate windings unless the ground loop circuit is below a predetermined resistance value and is electrically complete. A control relay is included in the circuit of the gate windings, the arrangement being such that the relay picks up and remains energized so long as the ground loop circuit is in good condition and deenergizes to perform a signal function or a power trip function upon the occurrence of ground circuit discontinuity or resistivity change above a given value. In addition, the bias windings are supplied through a full wave rectifier and to guard against false operation in case the rectifiers become faulty, a series relay may be provided in the bias winding circuit, with its contacts in the circuit of the control relay, so that during correct operation both relays have to be normally energized, thereby assuring that the system and the ground loop circuit are in good and effective operating condition.

in the drawings,

FIG. 1 is a schematic illustration of a tilting ladle furnace provided with a cover grounding loop circuit and a safety ground relay of the present invention which permits power flow to the heating element in the cover only when the ground loop circuit is connected to the cover;

FIG. 2 is a schematic diagram of circuits embodying the invention in a preferred form thereof;

FIG. 3 illustrates B-H graphs of the respective cores indicating flux excursions during gating in one core and resetting in the other core for, say, the positive half cycle of supply voltage;

FIG. 4 is a modification of the circuit of FIG. 2, and

FIG. 5 illustrates a further modification.

Referring to FIG. 1, numeral 10 indicates a refractory lined tilting ladle or pouring furnace for containing molten metal and 11 indicates a removable cover therefor which also has a refractory lining and carries a suspended electrical heating eiemen-t 12, thermocouple 13 and detachable or separable plug and receptacle type electrical connectors 14 and 15 for the heater and thermocouple circuits, respectively. The ladle or furnace is adapted to be tilted about pivots carried by pedestals 16 on opposite sides of a pouring spout (not shown) on the ladle for pouring of its molten metal contents, controlled tilting of the ladle being effected by the usual hydraulic cylinder 17. The apparatus thus briefly described is conventional and well-known, so need not be illustrated and described in detail, as also are the electrical connectors 14- and 15 and which may be of any suitable type having the requisite number of separable poles.

As indicated in FIGS. 1 and 2, the circuit of heating element 12 extends through connector El i, flexible cables or conductors 2%), contacts of a power relay PR and conductors Zil to a 440 volt AC. power supply source, represented by terminals 22. The elements of this circuit are of adequate size to handle the power transmitted therethrough. In addition to poles in the connector 14 to which the power circuit conductors are connected, the connector is provided with a ground pole which is electrically connected, as indicated at 23, to the metal shell of the cover 11 and the other end of this pole is connected to a flexible ground cable 24 which extends to and is electrically connected to an earth ground terminal, as indicated at 25. Likewise, the circuit of thermocouple 13 extends through connector and conductors 26 to the input terminals of a conventional high-low temperature controller 27 having the usual control switch contacts 28 connected to control the operation of the power relay PR, whereby the temperature in the ladle is main tained between preselected high and low levels. A ground pole in connector 15 is also connected, as at 29, to the shell of cover ill and the other end of this pole is connected to a flexible ground cable 50 which extends to and is electrically connected to ground terminal 25, although it may be connected to any other earth ground point, if desired. It will thus be seen that the cover is provided with two parallel grounds so arranged for personnel safety that an operator cannot disconnect both simultaneously and that these grounds form a low resistance ground loop circuit when both connectors 14 and 15 are in connected position. Both ground cables should be of adequate size to insure against mechanical damage and to provide low impedance for severe short-circuit current flow through either of them that might occur. The total impedance of the ground loop circuit should be extremely low because continuous monitoring of the ground loop circuit according to the present invention is predicated on the presence of a continuous and low impedance condition of such circuit, otherwise a power trip operation is effected.

In order continuously to test the condition of the ground loop circuit for continuity and low resistivity, two small saturable reactors or magnetic amplifiers 31 and 32 are provided to which the ground loop circuit is magnetically linked by passing one ground cable 24 through the open center of one reactor and passing the other ground cable in the same direction through the open center of the other reactor. Thereby the ground circuit constitutes a series connected, one turn control winding for the reactors. It is appropriate to have both reactors threaded by only one of the cables, if desired, provided connection of their windings in proper polarity relationship is effected. The center opening of each reactor is of sufiicient size to permit a 4/0, 600 volt insulated cable to be passed therethrough. Usually, 2/0 size cable of the required length may be used and thus will readily pass through the reactors, yet assure a total loop circuit impedance of less than 0.1 ohm; but, of course, the size of the reactors and ground circuit impedance can be varied by design over quite wide limits without departing from the principles of the invention.

The reactors are substantially duplicates and, respectively, comprise closed loop cores 3334, preferably toroids, of high permeability material, on which are wound load or gate windings 35-36 of about 2500 turns, for example, and bias windings 3738 of about 500 turns, for example. The two bias windings 37-38 are connected in series by conductor 39 and by conductors 40 and 41 are connected to the output terminals of a full wave bridge rectifier 4 2. The input terminals of the rectifier are connected to the secondary winding 43 of transformer T, whose primary 4-4 is connected to alternating current source terminals 45. The bias current supplied to windings 37-353 is preset to a fixed value by means of resistor 56 in the bias circuit and this resistor may be of a variable resistance type to permit adjustment of the bias current. Also included in the bias circuit is the coil of a sensitive relay SR shunted by a protective resistor 47, which relay is normally energized by the current flow in the bias circuit. In case of rectifier fault or opening of the bias circuit, relay SR deenergizes and its contacts 43 opens. These contacts are connected in one side of the gate circuit of the reactors and hence will disconnect them upon the occurrence of a fault in the bias circuit. The performance of the reactors is dependent on bias current constancy, so transformer T is of the regulating, constant potential type, or a substantially constant battery-voltage source may be used.

The gate windings 35-456 are connected in a closed loop circuit with diodes or rcctifiers 5$L'l. connected in series therewith and similarly polarized. A point 52 between the gate windings is connected by conductor 53 to one power supply terminal 45 and the other side of this gate circuit extends from point 54 between the rectifiers through conductor 55, contacts of relay SR, conductor 56, coil S7 of control or load relay CR and condoctor 53 to the other power supply terminal 45. Resistor 59 is connected across the coil of relay CR in order to load the reactors and improve the response time of the control relay. Normally open contacts 60 of control relay CR are in series with the control switch contacts 23 of the temperature controller 27 and obviously must be closed in or er to permit operation of the power relay PR. The rectifiers in the gate circuit cause operation of the reactors in the self-saturating doubler mode, each reactor being reset during the half-cycle period that the line voltage is applied through the rectifier and gate winding of the other reactor. Normally closed contacts 61 of relay CR may be connected to control the circuit of any suitable visual or audible alarm or signal device, here shown as a bell 62, and located at any desired point, to indicate a non-operating condition of the system.

In the operation of the system, the bias current I in the bias circuit biases the reactors to cut-off and also maintains relay SR energized, its contacts 48 maintaining the gating circuit closed. Relay CR in the gate circuit may energize or not, dependent on the condition of the ground loop circuit which constitutes a short-circuited control winding or control loop for the reactors and governs the output therefrom. Without this control winding being present, neither gate winding 35 nor So can saturate its core on its gating half cycle of the line voltage because of the opposing bias, and reactor output is kept below the pick up or threshold voltage value of relay CR. According to this invention, bias bucking ampere turns produced by current flow in the shorted control winding due to voltage induced therein responsively to flux change in the respective cores is utilized to control core reset and reactor output. This current is dependent on the condition of the ground loop circuit. Assuming the circuit to be continuous and of a resistance of about 0.1 ohm or less, which is exemplary and quite normal for the illustrated application, it has been found that the gate current exceeds the threshold value for the relay CR and it picks up and renders the power relay circuit operable under control of the temperature controller. If, however, should any part of the ground circuit be broken or become high resistant (above 0.1 ohm) by reason of looseness or corrosion, the relay CR will drop out and prevent operation of power relay PR.

In further explanation of the above operation, reference may be had to FIGS. 2 and 3 in which arrows indicate the magnetizing directions of the windings on the cores and the flux excursions in the cores, respectively, for gating of core 33 and resetting of core 34'- with an eifective ground loop circuit constituting the shorted control loop. It will be seen that the flux in core 33 is changing towards core saturation during the forward gate current pulse I through gate winding 35 and its series reca sence ti-fier t while blocking rectifier 51 has, in effect, disconnected gate winding 36 so it has zero gate current and its core 34 is being reset by the bias winding 38. The flux change in core 34, out of saturation, induces a voltage in the control loop and a small exciting current I flows therein in a direction to oppose the bias ampere turns and decrease the amount of flux reset. The core flux has reset to point A when the A.C. cycle reverses. Obviously, the impedance or ohms in the control loop circuit directly controls the exciting current I and the amount of flux reset; the bias current being constant or fixed. In core 33, voltage in the control loop caused by flux change toward saturation is, as indicated, in the direction to aid the voltage produced in the loop by the flux change in core 34 in controlling core reset. When the AC. voltage reverses, the same reset control action for core 33 occurs; the small exciting current I flowing in the control loop alternately in opposite directions preventing complete reset of either core. Thereby, the gate current pulses are enabled to effect saturation and firing of each reactor in turn so that control relay CR picks up and remains energized so long as continuity of the control loop remains intact and its resistance does not go above a given value, 0.1 ohm in this instance, and thus lower the control current. Such interruption or lessening of this circulating current in the control loop allows complete or greater reset of the cores by their bias windings and their gate windings will absorb most of the line voltage leaving insufiicient voltage to maintain energization of the control relay and it drops out. In consequence, its contacts 60 opens the power relay circuit and its contacts 61 close so as to cause operation of the alarm or signal device thereby warning that the ground circuit is not what it should be in its ability to carry fault current and, from the safety standpoint, prevent a rise of voltage above ground potential, of the grounded object.

It may be here mentioned that tests with fault current flow in the ground loop showed no influence on the normal protective operation of the reactor system.

The impedance values above mentioned are to be considered exemplary, as for the normal range of ground circuit resistance and within the range of a given reactor structure design, different impedance values can readily be accommodated by adjustment of the common bias current and by putting more ground loop turns on each reactor.

In FIG. 4, there is shown a typical power transmission arrangement which may include two remotely disposed tower structures 65 each carrying a pothead 66 between which extends power cable 67 buried underground. The cable may be a single or multi-conductor type with its metallic static sheath covered with the usual corrosion protective coating. It is highly desirable to detect corroded shields but obviously visual inspection is impossible and impractical. In this case, supervision of the sheath for continuity may be easily effected. As shown, adjacent to each pothead the sheath protective coating is scraped away to bare the metal sheath and such bared areas are suitably electrically connected through wire conductors 68 and 69, respectively, to the adjacent grounded tower structure. Resistor 70 of suitable ohmic value may be included in the conductor circuit 69 to limit endto-end circulating current through the sheath. The other conductor 63 is extended through the open center of two reactors suitably mounted in a weather-proof housing (not shown) and of essentially the same structure and circuits as above described for reactors 3'1-32, in fact, similar parts bear similar identifying numerals. The principal difference is that no power relay control circuit is utilized, so the control relay CR has normally closed contacts 61 wired to control the circuit of a remote signal 62 located at a power control center. In this circuit, since the straight-through passage of the ground wire 68 through the reactors reverses the polarity of the reset control circuit with respect to one reactor, the gate circuit to the gate winding of that reactor may be reversed to provide the necessary operating polarity relation.

In operation, the bias current is adjusted to accommodate for the total resistance in the grounded sheath circuit to enable firing or" the reactors so that relay CR is normally energized. Should, in time, corrosion of any part of the ground circuit occur and reduce or interrupt reset control current therethrough, the relay drops out and causes operation of the signalling device, whereupon the defect can be located and repairs made before substantial damage to the cable occurs.

Illustrating the versatility and utility of the invention, it is shown in FIG. 5 applied to a typical high voltage transformer station where it is customary for engineering rea sons to ground the neutral of at least one Y-connected power transformer. The power transformer, as shown, has the neutral of its Y-connected primary adapted for connection to ground through cable and disconnect switch 81 in the usual manner. A parallel ground conductor 82, connected to ground side of switch 81, is also connected to ground to provide a continuous ground loop circuit. This conductor may be of much smaller size than cable 80, if desired. Continuous tell-tale of the condition of the ground circuit is afforded by threading the conductors 3tl82 through the open-center reactors 83S4, respectively, of substantially the same structure and circuits shown in FIG. 2, whereupon control relay CR remains energized so long as the ground circuit remains intact and its resistivity does not increase and reduce the reactor output below the drop out value of the control relay. Upon such occurrence, the relay deenergizes and signals the fact by completing the circuit of a signal device 62 located at an attended control center.

It is to be understood that my invention is not limited to the precise constructions shown and described, but is capable of modifications by one skilled in the art within the invention, as set out in the appended claims.

What is claimed is:

1. A ground circuit tell-tale system comprising a saturable reactor core of high permeability material and closed loop form, a bias winding on said core connected to a source of substantially constant direct current voltage through a preset current controlling resistance for biasing said core to cut-off, a gate winding on said core connected through a halt-wave rectifier in series with a relay means operably responsive to gate current above a threshold value and a source of alternating current, means for controlling flux reset in said core and allow said gate winding to drive the core into saturation and cause the gate current to exceed said threshold value and maintain said relay means normally energized, said means comprising a continuous ground loop circuit including a conductor magnetically linked with and constituting a shortcircuited control winding of low impedance for said core, said control winding being excited responsively to reset flux change in said core and bucking said bias winding to prevent complete core reset in dependence on the continuity and sustained low resistivity of said ground loop circuit and electrically actuated means responsive to deenergization of said relay means for effecting tell-tale of said ground loop circuit.

2. A ground circuit tell-tale system comprising a saturable reactor core of high permeability material and closed loop form, a. bias winding on said core connected to a source of substantially constant direct current voltage through a preset current controlling resistance for biasing said core to cut-oil, a relay connected in series with said bias winding and normally energized by bias current to maintain its relay contacts in closed position, a gate winding on said core connected through a half-wave rectifier in series with a relay means operably responsive to gate current above a threshold value and a source of alternating current, means for controlling flux reset in said core and allow said gate winding to drive the core into saturaion and cause the gate current to exceed said threshold value and maintain said relay means normally energized, said means comprising a continuous ground loop circuit including a conductor magnetically linked with and constituting a short-circuited control winding of low impedance for said core, said control winding being excited responsively to reset flux change in said core and bucking said bias winding to prevent complete core reset in dependence on the continuity and sustained low resistivity of said ground loop circuit, and electrically actuated means responsive to deenergization of said relay means for effecting tell-tale of said ground loop circuit, said relay contacts being connected in series with said relay means and said gate winding to connect the same to said alternating current source only when said relay is energized.

3. A ground circuit supervisory system comprising two separate saturable reactors, each having a closed-loop core of high permeability material and a gate winding and a bias winding wound thereon, a bias circuit connecting in series circuit relation said bias windings, a preset bias current controlling resistance and a substantially constant direct current supply source for biasing said cores to cutoff, a gate circuit connecting each gate winding through a half-wave rectifier to a source of alternating current for gate current flow through the gate winding of one reactor and flux reset of the other reactor core at each half cycle of the AC. supply voltage, relay means series connected in said gate circuit and operably responsive to gate current above a threshold value, means for controlling core flux reset and enable said gate windings to drive their cores into saturation and cause the gate current to exceed said threshold value and maintain said relay means normally energized, said means comprising a low impedance ground loop circuit including a conductor magnetically linked with said cores and constituting a common control winding therefor, said control winding being excited responsively to flux change in said cores and opposing each bias winding during the reset interval to prevent complete core reset in dependence on the continuity and sustained low resistivity of said ground loop circuit, and electrically actuated means responsive to deenergization of said relay means for indicating the condition of said ground loop circuit.

4. A ground circuit supervisory system comprising two separate saturable reactors, each comprising a toroidal core of high permeability material, a gate winding and a bias winding wound thereon and providing an open center therethrough, an alternating current source, a rectifier having AC. input terminals and DC. output terminals, a regulating constant potential transformer having its primary connected to said A.C. source and its secondary connected to said AJC. input terminals, a bias circuit series connecting said bias winding through a preset variable current controlling resistance to said DC. output terminals for biasing said cores to cut-off, a gate circuit connecting each gate winding through a half-wave rectifier to said A.C. source for gate current flow through the gate winding of one reactor and flux reset of the other reactor core at each half cycle of the A.C. supply voltage, relay means series connected in said gate circuit and operably responsive to gate current above a threshold value, means for controlling core flux reset and permit said gate windings to drive their cores into saturation and cause the gate current to exceed said threshold value and maintain said relay means normally energized, said means comprising two ground conductors, each passing through the open center of one of said reactors, one electrically connected to the other at one end and both grounded at their other ends and constituting a short-circuited ground loop circuit of low impedance for said reactors, said loop circuit being excited responsively to flux change in said cores and bucking each bias Winding during the reset interval to prevent complete core reset in dependence on the continuity and sustained low resistivity of said ground loop circuit, and signalling means responsive to deenergization of said relay mean for indicating the condition of said ground loop circuit.

5. An electrical system as defined in claim 4, wherein a sensitive relay having contacts connected to control said gate circuit is series connected in said bias circuit for actuation responsively to bias current flow, and a resistor shunting the operating coil of said relay.

6. In an electrical control, a ladle furnace having a removable cover carrying an electrical heating element adapted to be connected through a detachable connector on said cover and contacts of a power relay to an electrical power source, a temperature controller operably responsive to a thermocouple carried by said cover and having switch contacts connected in series with an operating coil of said power relay and an AC. supply source for effecting on and off operation of said heating element, two separate saturable reactors of open center form, each comprising a toroid core with a gate winding and a bias Winding thereon, a first ground cable connected to said cover through said connector and passing through one reactor, a second ground cable passing through the other reactor and adapted to be detachably connected to said cover, said cables constituting a common, low impedance flux reset control winding for said reactors when in operative connection with said cover, a bias circuit including in series said bias windings, a preset bias current controlling resistance and a substantially constant direct current supply source for biasing said reactor cores to cut-0d, each gate winding being connected through a half-wave rectifier in series with the coil of a control relay operably responsive to gate current above a threshold value and said A.C. supply source for gating of one reactor and flux reset of the other reactor core at each half cycle of the AC. supply voltage, said control winding being excited responsively to flux change in said cores and bucking each bias winding during the reset interval to prevent full reset and enable the gate windings to saturate the cores on the gating intervals and cause the gate current to exceed said threshold value and maintain said control relay normally energized, and contact means controlled by said control relay and connected in circuit with the coil of said power relay for controlling power fiow to said heating element in dependence on the continuity and sustained low resistivity of said ground cables.

7. In an electrical control, a ladle furnace having a removable cover carrying an electrical heating element and a thermocouple, circuit means connecting said heating element through a detachable power connector on said cover and contacts of a power relay to an electrical power source, circuit means connecting said thermocouple through another detachable connector on said cover to a temperature controller operably responsive to said thermocouple and having switch contacts connected in series with an operating coil of said power relay and an AC. supply source for effecting on-otl operation of said heating element, two separate saturable reactors of open center form, each comprising a toroid core with a gate winding and a bias winding thereon, two ground cables individually connected to said cover through said connectors and passing through said reactors, respectively, and connected to ground, said cables constituting a common, low impedance flux reset control winding for said reactors when in operative connection with said cover, a constant potential regulating tran former having its primary connected to said A.C. supply source and its secondary connected through a full wave rectifier to said bias windings in series for biasing said reactors to cut-0E, a sensitive relay having an operating coil shunted by a resistor connected in series with said bias windings and normally energized by the bias current therethrough to maintain its relay contacts in closed position, each gate winding being connected through a half-wave rectifier to said A.C. supply source in series circuit relation with said closed relay contacts and the coil of a control relay operably responsive to gate current above a threshold value, said rectifiers being poled for gating of one reactor and flux reset of the other reactor core at each half cycle of the AC. supply voltage, said control winding being excited responsively to flux change in said cores and bucking each bias winding during the reset interval to prevent full reset and enable the gate windings to saturate the cores on the gating intervals and cause the gate current to exceed said threshold value and maintain said control relay normally energized, and contact means controlled by said control relay and connected in circuit with the coil of said power relay 19 for controlling power flow to said heating element in dependence on the continuity and sustained low resistivity of said ground cables and upon continued energization of said sensitive relay.

8. The system as defined in claim 3 wherein said ground loop circuit has an impedance of about 0.1 ohm or less and said relay means =deenergizes responsively to im edance increase above said ohmic value.

References ited in the file of this patent UNITED STATES PATENTS 2,810,526 Rogers Oct. 22, 1957 

1. A GROUND CIRCUIT TELL-TALE SYSTEM COMPRISING A SATURABLE REACTOR CORE OF HIGH PERMEABILITY MATERIAL AND CLOSED LOOP FORM, A BIAS WINDING ON SAID CORE CONNECTED TO A SOURCE OF SUBSTANTIALLY CONSTANT DIRECT CURRENT VOLTAGE THROUGH A PRESET CURRENT CONTROLLING RESISTANCE FOR BIASING SAID CORE TO CUT-OFF, A GATE WINDING ON SAID CORE CONNECTED THROUGH A HALF-WAVE RECTIFIER IN SERIES WITH A RELAY MEANS OPERABLY RESPONSIVE TO GATE CURRENT ABOVE A THRESHOLD VALUE AND A SOURCE OF ALTERNTING CURRENT, MEANS FOR CONTROLLING FLUX RESET IN SAID CORE AND ALLOW SAID GATE WINDING TO DRIVE THE CORE INTO SATURATION AND CAUSE THE GATE CURRENT TO EXCEED SAID THRESHOLD VALUE AND MAINTAIN SAID RELAY MEANS NORMALLY ENERGIZED, SAID MEANS COMPRISING A CONTINUOUS GROUND LOOP CIRCUIT INCLUDING A CONDUCTOR MAGNETICALLY LINKED WITH AND CONSTITUTING A SHORTCIRCUITED CONTROL WINDING OF LOW IMPEDANCE FOR SAID CORE, 